1. Field of the Invention
The present invention relates to a solid-state imaging device, and more specifically relates to a circuit configuration of a unit pixel. The imaging device of the present invention is used, for example, to a CMOS image sensor.
2. Description of the Related Art
A plurality of unit pixels is arranged in an imaging area of the CMOS image sensor. Each unit pixel includes a photodiode, a floating diffusion, a reading transistor, an amplification transistor, a reset transistor, and a selection transistor. The photodiode photoelectrically converts incident light. The reading transistor performs reading of accumulated electric charges from the floating diffusion. The amplification transistor amplifies the signal of the floating diffusion to output the amplified signal to a vertical signal line. The reset transistor resets a gate electric potential (an electric potential of a floating diffusion). The selection transistor selects the unit pixels to control an operation of the amplification transistor.
In general, it is well known for the CMOS sensor that a thermal noise and 1/f noise are conspicuous. These noises are called a dark state random noise. To improve the dark state random noise, it is effective to raise a signal level of the floating diffusion in a preliminary step of an occurrence of the random noise. The voltage amplitude out(v) of the floating diffusion is given by an equation of out(v)=(e/CFD)×input(q). Here, “e” is an electric charge amount, CFD is capacitance of the floating diffusion, and input(q) is the number of signal electric charges. To enlarge the voltage amplitude of the floating diffusion and reduce the dark state random noises, it is effective to reduce the CFD of the capacitance of the floating diffusion. A conversion gain Gain of the unit pixel is given by an expression of Gain=Out(v)/input(q).
FIG. 1 shows a figure illustrating a reset operation and a reading operation in a case in which a signal electric charge amount accumulated in the photodiode PD is relatively small in a generic COMS sensor. That is, by turning on the reset transistor RST to perform a reset operation, the electric potential of the floating diffusion FD immediately after the reset operation is set to the same electric potential level as that of a drain. Next, when turning on the reading transistor RD, since a signal electric charge amount accumulated in the photodiode PD is relatively small, it is possible to transfer the signal electric charges to the floating diffusion FD even if the capacitance CFD of the floating diffusion FD is designed in a small size.
FIG. 2 shows a view illustrating a reset operation and a reading operation in a case in which the signal electric charge amount accumulated in the photodiode FD is relatively large in a generic CMOS sensor. That is, turning on the rest transistor RST to perform the reset operation allows the electric potential of the floating diffusion immediately after the reset operation to be set to the same electric potential level as that of the drain. When turning on the reading transistor RD at the next time, since the signal electric charges accumulated in the photodiode PD becomes an over amount, it is impossible to transfer all of the signal electric charges to the floating diffusion FD. A remnant Lag of the signal electric charges generates in the photodiode PD.
A technique adding gate capacitance to the floating diffusion in the unit pixel, which makes the capacitance of the floating diffusion variable, is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-165754. However, in this technique, even when tuning off the gate capacitance added for high sensitivity, parasite capacitance between the gate and the floating diffusion cannot be perfectly made zero. Thereby, in comparison with a case where the floating diffusion capacitance is not made variable, the floating diffusion capacitance increases, and the conversion gain decreases. As a result, a dark state random noise characteristic is deteriorated.
Meanwhile, a technique which increases the signal electric charge treatment amount by adding capacitance to a photodiode through a MOS transistor in a unit pixel is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-77737. However, in this technique, the reading transistor is not existed, and the photodiode and the newly added capacitance are connected to a gate of an amplification transistor. Thereby, a conversion gain is lowered. As a result, dark state random noise characteristics are deteriorated.
A technique increases a signal electric charge amount by adding capacitance to the photodiode through a MOS transistor is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-245522. However, in this technique, in comparison with the conventional unit pixel, since it is needed to add circuit elements of total four of two transistors and two capacitors, shrink of the pixel size is made difficult.
As described above, in the conventional solid-state imaging device, the dark state random noise and the signal electric charge treatment amount are in a tradeoff (inconsistent) relationship, it is difficult to increase the signal electric charge treatment amount while maintaining the low noise in the dark, the improvement in this problem is desired.